Issues with Zoysiagrass Lawns

Published: April 30, 2012

Power-rakes can be rented locally. Set blade depth to cut through thatch layer.

Spring came very early this year and warm-season grasses such as zoysiagrass came out of winter dormancy about one month ahead of schedule. Pat Guinan, MU Extension Climatologist, indicated that March temperatures were 12 to 14 degrees higher than normal and placed this March as the warmest on record. He also commented that if March's temperatures were laid over April of this year, the monthly average for April would still be 5 to 6 degrees above normal.

With this early spring, zoysiagrass began to green-up and the questions began to roll in about the fate of zoysiagrass. Zoysiagrass lawns are not looking well. It seems the St. Louis area has been hit the hardest and it dominates as the number one question about home lawns, "What is wrong with my zoysiagrass lawn? It looks dead."

We are presently looking at three possible scenarios – large patch, hunting billbug, and chinch bug. If chemical pesticide applications are necessary to combat these problems, it is recommended to use a certified applicator or contact your county extension agent for product selection and use guidelines.

Large patch is the number one biotic problem that affects zoysiagrass on an annual basis. It is indiscriminate in its occurrence, and will damage lawns or golf courses with similar voracity. The disease is caused by the fungal pathogen, Rhizoctonia solani, and is a close relative to the identically named pathogen that causes brown patch on tall fescue. Large patch symptoms can occur in the fall but are most severe now in the spring, when zoysiagrass is slow growing due to cool temperatures and is coming out of winter dormancy (i.e. hibernation). Actual infection of the fungal pathogen, however, occurs in the early fall when the fungus dives down to the base of the plant inside the leaf sheaths. In active outbreaks, leaves on the outer margins of patches will "fire" and turn a brilliant orange color that is most vivid in the morning or after a rain event. As the name implies, large patch symptoms can be quite grand in nature, with patches ranging from 6 inches to many, many feet in diameter. Some extreme outbreaks can even be observed with satellite imagery!

Large patch affects zoysiagrass lawns in spring and fall. Inset: Leaves along large patch margins “fire” and turn orange when the disease is active.

De-thatching can be more damaging to a lawn; however it will remove thatch more quickly in a severe thatch problem.

Large patch is not easy to control, and researchers are still learning important aspects of the disease cycle in order to develop more effective management practices. It is critical that timing of fertilization and agronomic practices is done in accordance with when zoysiagrass is actively growing. Nitrogen fertilization in the fall or spring when zoysia growth is slow, tips the scales squarely in favor of the large patch pathogen and will result in severe outbreaks. Similarly, aerification and thatch removal practices should be limited to the hot summer months. So, in other words, don't do anything to zoysia unless you are actively sweating while doing it. Because the pathogen depends on leaf moisture, over-irrigation and poor drainage will also result in more severe large patch outbreaks.

Chemical control of large patch should be limited to areas that have a history of the disease. If fungicide control is necessary, the application must be timed preventively when infection is taking place not when symptoms occur. Because of this, fungicide applications should be made in the fall and recent research suggests earlier may be better with early to mid September when soil temperatures dip to below 70°F. To minimize fungicide use, it is possible to map out the diseased areas on lawns now, and only specifically apply fungicide to these areas this fall. Fungicide applications made to lawns now in the spring will protect your healthy grass from large patch expansion, but will not magically cure zoysia that has already been infected.

Fig.B. Chinch bug damage on zoysia.

Fig. A. Chinch Bug.

Chinch bug and hunting billbug outbreaks were also noted on several lawns in the St. Louis and Jefferson City area in the last two years.

Chinch bugs (Blissus spp.) are the most damaging insect pest in zoysiagrass. Unlike large patch, chinch bug damage occurs in the hot, dry summer months and most closely resembles drought damage. Affected areas are solid, not patchy, and will usually start on one side of the lawn and progress throughout as the chinch bug population builds and moves. Damage is most severe along lawn boundaries, particularly concrete driveways and sidewalks. The easiest method to detect chinch bug damage is to pull up damaged zoysia along these boundaries and look for the scattering fast, small 3/16-inch black bugs. If chinch bugs are occurring, there is no recourse but the use of a curative insecticide to eliminate the problem. Because occurrence is sporadic from year to year, preventive insecticide applications targeted for chinch bugs are not recommended in this area.

Hunting Billbug.

Hunting Billbug damage on zoysia.

Hunting billbugs (Sphenophorus venatus vestitus) have been also been found sporadically in Missouri over the last two years. Unlike chinch bugs, hunting billbugs are more anonymous and elusive. Because of this little is known of hunting billbug biology. Adults are reddish brown-black, 1/2-inch long, have a curved snout, and are most active during the night and early morning hours. Billbugs are thought to overwinter as adults, and lay eggs in grass stems/leaf sheaths in mid-late spring. Billbug larvae, which unlike annual white grubs are legless, hatch and feed by boring into lower leaf stems. Larvae become larger and also feed on stolons, which are left characteristically hollowed out in early summer. At this point, zoysia will easily pull away from the soil, and symptoms will occur as yellow areas that eventually brown and die out, resembling drought damage. Monitoring both adult and larval hunting billbug activity is difficult. The most effective method for detection of adult activity (which should be occurring soon) is creating several pitfall traps in the lawn by digging a few holes and placing plastic cups level to the soil surface. Adults will fall into the cup overnight and can be counted over a few days period. Early larval stages are small and difficult to detect, but larger larvae in July can be observed by pulling zoysia away from the soil. In areas where hunting billbug damage has been identified, a preventive long lasting insecticide application should be applied in late May – early June (same time frame for annual white grubs) to target both adults and larvae.

Beyond the winter woes and pest problems, zoysiagrass maintenance can be very simple. However, one practice that is often over-looked and contributes to the pest problems mentioned above is thatch control.

Thatch is the organic layer above the soil line and should not exceed ½ inch.

Thatch is a layer of decomposed and partially decomposed roots, stems, stolons and rhizomes. Thatch appears as a distinct horizontal layer of brown spongy or felt-like material just above the soil surface. Zoysiagrass is prone to thatch accumulation because of its rapid growth and thick network of stolons and rhizomes. Clippings from infrequent mowing will not decompose properly resulting in some initial thatch build up. In time, thatch can build to depths greater than 1 ½ inches. Use a hand trowel to cut a plug from the lawn at least 4 inches deep. Thatch layers can usually be seen in the profile of that plug. Lawns having a thatch layer exceeding ½ inch should be de-thatched. Maintaining thatch in a zoysiagrass lawn will reduce your potential for pest issues like Large Patch disease and insects. Power-rakes or vertical mowers can be rented locally and will accomplish this task.

If thatch is greater than one inch, do not attempt complete removal in one year. Instead, remove the thatch over a period of two or three years. Intensive coring should also be considered since this causes much less damage to the turf than does power raking or vertical mowing. Coring can be achieved with walk-behind units that are rented locally as well. Soil plugs are removed from the root-zone and placed on the surface of the lawn. Soil from core plugs will eventually melt away spreading soil and soil microbes into the thatch layer. Core aeration, done annually, can help maintain thatch with less damage.

Thatch buildup can be minimized through good cultural practices, including the following:

Mow grass regularly at the recommended height to maintain vigor and to avoid shock.

Water deeply and only as needed.

Core aerification helps to reduce thatch build-up. Aerification also reduces soil compaction and improves movement of fertilizers, air and water into the root-zone.

NOTE: Avoid core aeration with the presence of active Large Patch disease. Aeration will promote the spreading of this disease.

Fertilizing
Established zoysiagrass requires less fertilizer than many other species for healthy, attractive turf. Seasonal totals of 2 pounds of nitrogen per 1,000 square feet are ample. Excessive or untimely fertilizer applications can lead to problems such as fewer roots, more thatch, diseases, and more top growth that requires increased mowing.

For best results, soil testing is recommended before fertilizing. Soil test will indicate any major nutrient deficiencies and the acidity or alkalinity (pH) of the soil. Slightly acidic soil pH (6.0-6.5) is best. Lime should be applied only if the pH is less than 6.0.

Established zoysiagrass should be fertilized from May through August. Early spring (March/April) fertilization benefits weeds and promotes premature top growth before root development begins. Late fertilization (September or later) may interfere with the natural hardening (reducing cell moisture) process before winter.

For routine maintenance where soil tests indicate no major deficiencies, use a lawn fertilizer with approximate nitrogen (N):phosphorus (P):potassium (K) ratios of 3:1:1, 4:1:1 or 4:1:2. A 16-4-8 fertilizer has a 4:1:2 N:P:K ratio.

Where soil test indicates low phosphorus or potassium levels use a fertilizer with a ratio that more closely approximates a 1:1:1 or 2:1:1.

NOTE: All fertilizer containing nitrogen should be avoided with the presence of active Large Patch disease.

Mowing
Zoysiagrass is mowed at a shorter cutting heights (1 ½ - 2 inches) than Kentucky bluegrass or tall fescue (3 – 4 inches). In spring, zoysiagrass may be mowed at the lowest setting on your mower to remove dead leaf tissue. This increases the green-up rate and allows easier and more uniform mowing during the growing season. The mowing height should be raised in September by 1/2 to 1 inch in preparation for fall.

When mowing, never remove more than one-third of the leaf blade at any one time. Clippings need not be collected if they do not remain as clumps on the lawn surface. Remove excessive clippings that accumulate if mowing is delayed.

Watering
Zoysiagrass is a drought-tolerant lawn grass that requires less water than Kentucky bluegrass or tall fescue to remain green and actively growing during the summer months. Watering usually is not necessary except during prolonged dry periods. However, keep in mind that Missouri soils are heavy with clay and shallow. Root development in spring may be limited; therefore creating situations where zoysiagrass can be very drought susceptible incurring damage.

Other cultural practices mentioned above, such as thatch control, fertilization, and mowing can go a long way toward building a drought-tolerant lawn. When watering, follow these simple rules:

Water in early morning to reduce evaporative losses, provide better distribution of your irrigation and reduce disease incidence by removing heavy dew.

Water deeply, wetting the soil to a depth of 4 to 6 inches. Only apply as much irrigation as the soil can absorb. Many Missouri soils have infiltration rates between o.25 and 0.50 inches per hour. Always avoid puddles and runoff.